In any area where radioactive materials are handled, it is imperative, both for the protection of personnel and to avoid contamination of the environment, to continuously monitor personnel, air vents, smoke stacks, surfaces, equipment, and clothing to prevent the release of radioactive contamination. Alpha contaminants, such as plutonium and uranium, are particularly difficult to detect because plutonium primarily emits alpha radiation, and alpha radiation has very limited penetration in air. Alpha particles from typical contaminants travel only a few inches in air. It is because of this characteristic that prior alpha detectors have been useful only when used in close proximity to the point of possible radioactive emission.
In the past, several instrument designs have been utilized to detect alpha radiation. Among these are GM tubes, ionization chambers, count rate detectors, and scintillation or gas flow proportional probes. While these instruments are capable of detecting alpha particles, they do so by directly detecting incident radiation, and must be within an inch of the source of the radiation. Also, these conventional alpha particle detectors can only scan an area approximately equal to the size of the detector, and are of little value in detecting radiation in a high gas flow situation.
As used herein, the terms "long range," or "long distance," when referring to the detection capabilities of the present invention, shall mean detection from a range or distance of more than one (1) inch from the source of alpha radiation.
The primary reason for an alpha particle's short flight path in gas is its collision with air or other gas molecules. In almost all of these collisions, various of the molecular species in air or a gas are ionized. These ions, referred to herein as "gas ions," have a sufficiently long lifetime that they may be transported by mass flow of the surrounding air, or by the direct attraction of an electric field, and detected at distances much greater than the penetration distances of the original alpha particles. That is, the gas ions thus created have a longer life and area of influence than the alpha particles that created them. These are the ions that are detected by the present invention. The fact that the gas ions have a longer range than the alpha particles relieves the necessity for having a detector moved in close proximity over a person or equipment in order to detect the presence of alpha radiation.
The present invention provides an alpha monitor for use with high gas flows. It is based on technology which is contained in several U.S. Patents which disclose various devices for the long range detection of alpha particles. The first is U.S. Pat. No. 5,184,019, issued Feb. 2, 1993, for a Long Range Alpha Particle Detector. The second is U.S. Pat. No. 5,194,737, issued Mar. 16, 1993, for Single and Double Grid Long Range Alpha Detectors. The third is U.S. Pat. No. 5,187,370, issued Feb. 16, 1993, for Alternating Current Long Range Alpha Particle Detectors. The fourth is U.S. Pat. No. 5,281,824, issued Jan. 25, 1994, for Radon Detection. The fifth is U.S. Pat. No. 5,311,025, issued May 10, 1994, for Fan-less Long Range Alpha Detector. Another recently filed application bears Ser. No. 08/833020, filed Nov. 1, 1994, and is entitled "Event Counting Alpha Detector." Still another recently filed application bears Ser. No. 08/382,333, filed Feb. 1, 1995, and is entitled "Background Canceling Surface Alpha Detector." As previously described, the principle underlying each of these patents and patent applications is that alpha particles, although themselves of very short range in air, ionize various of the molecular species in air. The present invention modifies this apparatus to provide for reliable detection of alpha radiation from high gas flows, such as through an air vent or a smokestack.
The fact that long range alpha detectors, as described in the above-referenced patents and application, can detect alpha radiation at a considerable distance from its point of emanation allows for monitoring of contamination in several areas which are extremely difficult or even impossible for current detectors. However, these previous detectors would lose many ions if the gas flow velocity exceeds a certain level. The current invention is able to overcome this problem through the use of multiple signal collectors positioned parallel to the gas flow, the collectors being of alternating polarity.
It is therefore an object of the present invention to provide apparatus capable of the detection of alpha radiation in a high flow of air or other gas.
It is another object of the present invention to detect gas ions created by collision with alpha particles of both positive and negative polarities.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.